A backup system that has a secondary storage apparatus for backing up data, in preparation for a malfunction or the like of a primary storage apparatus, is known. As a backup method of the data in the backup system, for example, the method disclosed in PTL 1 is known.
According to this backup method, a mode for storing backup data in a data volume on the primary side to a data volume on the secondary side may be a synchronous mode (synchronous remote copy) that allows the data volumes on the primary side and the secondary side to be synchronous with each other, or an asynchronous mode (asynchronous remote copy) that allows the data in the data volume on the secondary side to asynchronously follow the data in the data volume on the primary side. According to this backup method, a first storage apparatus acts as a main body to execute data transfer in both of the synchronous mode and the asynchronous mode.